Dahlias provide a warning of frost

A recent overnight frost blackened our dahlias’ foliage causing it to collapse, so the garden salvage operations we began in October need to be stepped up.

Near the coast, the temperature falls incrementally in late autumn, from a morning of crisp white grass on the lawn to tips of soft green plants suffering damage.

Then comes the dahlia warning, followed by rock-hard ground and water like a stone, in the words of the song, prompting a sense of urgency right now.

We are fortunate, though. A friend, Jim, who lives out west and much closer to the Cheviots, has no such warning. One day it’s short sleeves, and the next can see hard frost.

Not digging up the dahlia tubers for winter storage under cover until frost has touched them is a popular approach, but there are occasions when you have to question whether such guidance is right for you. Do you need to dig them up at all?

Increasingly, we’re leaving tubers of small decorative and bedding types in the garden over winter to avoid the time-consuming alternative of storing them.

Another reason is the low cost of replacement should the mother of all frosts take them. A packet of mixed bedding dahlias costs little more than £2 and, with spring sowing indoors, can be flowering the first summer.

We planted some plugs of Bishop’s mixed dahlias a year or two ago for their bright foliage and blooms.

At the end of year one, the tubers were lifted and boxed up in compost, then transferred to the greenhouse so stem cuttings could be rooted from shoots the following March. This was duly achieved, but new shoots also arose from the spot in our border where the parent plants had grown.

I have not dug up the Bishop’s tubers since, but I take the precaution of spreading mulch over the site as extra protection against prolonged ground frost. Had lots of money been spent on special dahlia varieties, I might well be singing a different tune and lifting them with propagating more in mind.

Once they’re dug up, there is more work than at first meets the eye to keep them sound for next year. It’s advisable to wash away all of the attached soil that might harbour disease.

Then comes the drying-off process, which involves standing them upside down on greenhouse benching, exposed to the air in daytime but covered with fleece on colder nights.

Thick, green stubs of stems left for ease of handling can hold lots of residual moisture, posing a risk of rotting in store.

Show exhibitors of yore used to drill a hole down through stem to base to drain this off, then dust with flowers of sulphur.